MRNA iron-response elements

The existence of an iron-responsive cis-acting element in plant ferritin genes has been indicated in a series of studies. For example, iron-dependent induction of ferritin synthesis coincided with increased transcription and accumulation of ferritin mRNA [3]. Moreover, insertion of the animal IRE (mRNA iron responsive element) into soybean ferritin mRNA did not have much effect on the translation of the mRNA [4]. In addition, regulation of soybean ferritin expression in nodules [5] appeared to be posttranslational [6]. Finally, no IRE sequence was found in ferritin gene sequences from either maize or soybeans [7]. In fact, the organization of plant... 

Leedman, P., Stein, AR., Chin, WW., Rogers, JT., Thyroid hormone modulates the interaction between the iron regulatory protein and ferritin mRNA Iron-responsive elements. J. Biol. Chem., 1996. 271 p. 12017-12023. 

Synthesis of the transferrin receptor (TfR) and that of ferritin are reciprocally linked to cellular iron content. Specific untranslated sequences of the mRNAs for both proteins (named iron response elements) interact with a cytosolic protein sensitive to variations in levels of cellular iron (iron-responsive element-binding protein). When iron levels are high, cells use stored ferritin mRNA to synthesize ferritin, and the TfR mRNA is degraded. In contrast, when iron levels are low, the TfR mRNA is stabilized and increased synthesis of receptors occurs, while ferritin mRNA is apparently stored in an inactive form. This is an important example of control of expression of proteins at the translational level. 

Figure 18.14 Evidence for an iron-responsive element in the 5 -UTR of APP mRNA. APP 5 -UTR sequences were computer-folded to generate the predicted RNA stem. (From Rogers et al., 2002. Reproduced by permission of the Journal of Biological Chemistry.)...

SELEX has also allowed the characterization of the RNA hairpin, which constitutes the iron responsive element (IRE) recognized by the iron regulatory factor (IRF) protein to post-transcriptionally regulate translatability and decay of mRNAs involved in iron import and storage in eukaryotic cells (Henderson et al., 1994). 

Iron regulatory proteins (IRPs) regulate the cellular iron level in mammalian cells. IRPs are known as cytosol mRNA binding proteins which control the stability or the translation rate of mRNAs of iron metabolism-related proteins such as TfR, ferritin, and 5-aminolevulinic acid synthetase in response to the availability of cellular iron [19-21] after uptake [5]. The regulatory mechanism involves the interaction between the iron-responsive element (IRE) in the 3 or 5 untranslated regions of the transcripts and cytosolic IRPs (IRP-1 and -2). IRP-1 is an iron-sulfur (Fe-S) protein with aconitase activity containing a cubane 4Fe-4S cluster. When Fe is replete, IRP-1 prevails in a 4Fe-4S form as a holo-form and is an active cytoplasmic aconitase. As shown in Fig. 3, when Fe is deplete, it readily loses one Fe from the fourth labile Fe in the Fe-S cluster to become a 3Fe-4S cluster and in this state has little enzymatic activity [22, 23]. 

Thomson AM, Rogers IT, Leedman PJ. Iron-regulatory proteins, iron-responsive elements and ferritin mRNA translation. Int. J. Biochem. Cell. Biol. 1999 31 1139-1152. 

Consider ferritin first. Ferritin mRNA includes a stem-loop structure termed an iron-response element (IRE) in its 5 untranslated region (Figure 31.38). This stem-loop binds a 90-kd protein, czAlsd m IRE-bindingprotein (IRE-BP), that blocks the initiation of translation. When the iron level increases, the IRE-BP binds iron as a 4Fe-4S cluster. The IRE-BP bound to iron cannot bind RNA, because the binding sites for iron and RNA substantially overlap. Thus, in the presence of iron, ferritin mRNA is released from the IRE-BP and translated to produce ferritin, which sequesters the excess iron. 

Figure 31.38. Iron-Response Element. Ferritin mRNA includes a stem-loop structure, termed an iron-response element (IRE), in its 5 untranslated region. The IRE binds a specific protein that blocks the translation of this mRNA under low...

In eukaryotes, genes encoding proteins that transport and store iron are regulated at the translational level. Iron-response elements, structures that are present in certain mRNAs, are bound by an IRE-binding protein when this protein is not binding iron. Whether the expression of a gene is stimulated or inhibited in response to changes in the iron status of a cell depends on the location of the IRE within the mRNA. 

The structure of the IRE that occurs in the sequence of ferritin mRNA appears in Figure 10.34. The iron response element is a small region of RNA, and it is distinguished in that it spontaneously folds upon itself to form a hairpin shape. The ribonucleotides of RNA follow similar base-pairing rules as in DNA. In RNA, guanine binds to cytosine, and adenine binds to uracil. It is accurate to state that within the hairpin the RNA occurs as double-stranded RNA. To repeal, one might note that DNA contains a sequence of DNA bases that is used to code for the iron response clement, but these DNA bases do not bind the IRF... 

Iron response elements consist of short stretches of ribonucleic acid, and they occur in the mRNA coding for ferritin, transferrin receptor, and 8-aminolevulinic acid s)mthase. Each IRE can be bound by an IRP. Several different IRPs exist. Surprisingly, one of the IREs is quite similar to aconitase, the Krebs cycle enzyme (Kim et ah, 1996 Toth and Bridges, 1995 Henderson et ah, 1996). All IRPs contain an iron binding site. This site consists of several residues of cysteine. To be more specific, the sulfhydryl groups of cysteine residues function to bind iron atoms. 

Heme synthesis is controlled by a regulatory negative feedback loop in which heme inhibits the activity of fer-rochelatase and acquisition of iron fi om the transport protein transferrin. The decrease in iron acquisition leads to a decrease in iron uptake into the cell with subsequent decrease in 8-aminolevulinic acid and heme production. Iron deficiency and increased erythropoietin synthesis lead to the combination of the iron regulatory proteins with the iron-responsive elements in the transferrin receptor protein messenger ribonucleic acid (mRNA). This combination in turn leads to protection of the mRNA from degradation with subsequent increased uptake of iron into erythroid cells because of the increased expression of transferrin receptors on the cell membrane. 

Kato J, Fujikawa K, Kan da M, Fukuda N, Sasaki K, Takayama T, et al. A mutation, in the iron responsive element of H ferritin mRNA, causing autosomal dominant iron overload. Am J Hum Genet 2001 69 191-7. 

Total-body iron of a 70-kg adult is about 4.2-4A g. The distribution of iron in various body compartments is given in Table 29-1. The key players of iron metabolism include iron-responsive elements of appropriate mRNAs, iron regulatory proteins divalent metal transporter 1, major histocompatibility complex (MHC) class I-like protein designated as HFE protein, d2-microglobulin, transferrin, transferrin receptor, and ferritin. 

This mRNA has a set of iron-response elements (IREs) in its 3 untranslated region. The binding of the IRE-binding protein to these elements stabilizes the mRNA but does not interfere with translation,... 

Fig. 1.45 Regulation of the stability of the mRNA ofthe transferrin receptor by Fe3+. The translation of the mRNA ofthe transferrin receptor (TFR) is subject to regulation by the Fe concentration. Fe exerts its regulatory effect via the iron regulatory protein typel (IRP1). The IRP1 binds to a control segment at the 3 terminal region ofthe TFR mRNA, known as the iron responsive element (IRE). Binding of IRP1 to a hairpin structure of the IRE stabilizes the mRNA ofthe transferrin receptor and...

Control of intracellular iron concentrations by the iron-response element-binding protein (IRE-BP) is an elegant example of a single protein that regulates the translation of one mRNA and the degradation of another. When intracellular iron stores are low, this dual control system operates to increase the level of free Iron Ions available for Iron-requiring enzymes when Iron Is In excess, the system operates to prevent accumulation of toxic levels of free Ions. It Is one of the simplest and best-understood examples of protein-mediated translational control. 

---